Adaptive variable-length coding and decoding methods for image data

ABSTRACT

An adaptive variable-length coding/decoding method performs an optimal variable-length coding and decoding depending on an intra mode/inter mode condition, quantization step size and a current zigzag scanning position, such that a plurality of variable-length coding tables having different patterns of a regular region and an escape region according to statistical characteristics of the run level data are set. One of the variable-length coding tables is selected according to mode, quantization step size and scanning position, and the orthogonal transform coefficients according to the selected variable-length coding table are variable-length-coded.

This is a Divisional of application Ser. No. 09/654,939 filed Dec. 22,2000, which is a Divisional of U.S. application Ser. No. 09/638,796filed Aug. 11, 2000, which is a reissue of U.S. Pat. No. 5,793,897,issued Aug. 11, 1998. The entire disclosures of the prior applicationsare considered part of the disclosure of the accompanying divisionalapplication and are hereby incorporated by reference.

More than one reissue application has been filed for the reissue of U.S.Pat. No. 5,793,897 filed Nov. 3, 1995. The reissue applications areapplication Ser. Nos. 09/638,796 filed Aug. 11, 2000, now U.S. ReissuePat. RE39,167, 09/654,939 filed Aug. 31, 2000, 11/017,697 filed Dec. 22,2004, 11/017,698 filed Dec. 22, 2004, 11/416,183 filed May 3, 2006,11/416,312 filed May 3, 2006, 11/738,415 filed Apr. 20, 2007, which isthe present application, and 11/738,419 filed Apr. 20, 2007. ApplicationSer. No. 09/638,796 is a reissue of U.S. Pat. No. 5,793,897. ApplicationSer. No. 09/654,939 is a divisional application of U.S. Reissued Pat.RE39,167. Application Ser. No. 11/017,697 is a divisional application ofSer. No. 09/654,939. Application Ser. No. 11/017,698 is a divisionalapplication of Ser. No. 09/654,939. Application Ser. No. 11/416,183 is adivisional application of Ser. No. 11/017,697. Application Ser. No.11/416,312 is a divisional application of Ser. No. 11/017,698.Application Ser. No. 11/738,415, which is the present application, is adivisional application of Ser. No. 09/654,939. Application Ser. No.11/738,419 is a divisional application of Ser. No. 09/654,939. Theentire disclosures of U.S. Pat. No. 5,793,897, U.S. Reissued Pat.RE39,167, application Ser. No. 09/654,939 are hereby incorporated byreference.

TECHNICAL FIELD

The present invention relates to adaptive variablelength coding anddecoding methods for digital image data, and more particularly, toadaptive variable-length coding and decoding methods which improvecompression efficiency of transmission data by performingvariable-length coding and decoding adaptively, according to statisticalcharacteristics of image data.

BACKGROUND ART

Recently, in an apparatus for transmitting and receiving video and audiosignals, a method by which the video and audio signals are coded to bedigital signals to then be transmitted or stored in a memory and thedigital signals are decoded to then be reproduced, has been widelyadopted.

However, in the case of coding a video signal into digital data, thedata quantity is large. Thus, in order to decrease the overall dataquantity by removing redundant data contained in the digital videosignal, discrete cosine transform (DCT) coding, differential pulse codemodulation (DPCM), vector quantization, or variablelength coding (VLC)should be performed.

FIG. 1 is a schematic block diagram of a general coding system for imagedata. The apparatus includes means 11 and 12 for performing a DCTfunction with respect to an N×N block and for quantizing DCTcoefficients, means 13 and 14 for variable-length-coding the quantizeddata and for further compressing data quantity, and means 15, 16, 17,18, 19, A1, A2, SW1 and SW2 related to the inverse quantization and DCToperations with respect to the quantized data to then perform a motioncompensation, which codes image data in an intra mode or inter mode.

FIG. 2 is a schematic block diagram of a general decoding system forimage data. The apparatus decodes and reproduces the image data coded bythe coding system shown in FIG. 1.

The operation of the coding and decoding systems respectively shown inFIGS. 1 and 2 will be briefly described.

In FIG. 1, the video signal input through an input port 10 becomes asignal of a frequency domain in the units of N×N blocks in DCT 11, wherealthough the magnitude of a block is generally N₁×N₂, it is assumed thatN₁=N₂=N, for the sake of convenience. The energy of transformcoefficients is chiefly concentrated in a low frequency domain. Datatransforms for each block are performed by a discrete cosine transform.Walsh-Hadamard transform, discrete Fourier transform, or discrete sinetransform method. Here, the transform coefficients are obtained by DCToperation.

Quantizer 12 changes the DCT coefficients into representative values ofa constant level through a predetermined quantization process.

Variable-length encoder 13 variable-length-codes the representativevalues using their statistical characteristics, thereby furthercompressing the data.

Meanwhile, a quantization step size Q_(ss), which is varied depending onthe state (a fullness) of a buffer 14 wherein the variable-length-codeddata is stored, controls quantizer 12 to thereby adjust a transmissionbit rate. The quantization step size Q_(ss) is also transmitted to areceiver side, to be used in a decoding system.

Also, in general, there are many similar portions between consecutivescreens. Therefore, in the case of a screen having motion, a motionvector MV is obtained by estimating the motion, and data is compensatedusing the motion vector MV. Then, a differential signal betweenadjacently positioned screens becomes very small, thereby allowingtransmission data to be more compressed.

In order to perform such motion compensation, an inverse quantizer (Q⁻¹)15 shown in FIG. 1 inverse-quantizes the quantized data output fromquantizer 12. Thereafter, the inverse-quantized data isinverse-DCT-operated in an inverse DCT means (DCT⁻¹) 16 to then be avideo signal of a spatial domain. The video signal output from inverseDCT means 16 is stored in a frame memory 17 in frame units. Motionestimator 18 searches a block having the most similar pattern to that ofan N×N block of input port 10 among the frame data stored in framememory 17 and estimates the motion between blocks to obtain a motionvector MV. The motion vector MV is transmitted to a receiver side to beused in a decoding system and is simultaneously transmitted to a motioncompensator 19.

Motion compensator 19 receives the motion vector MV from motionestimator 18 and reads out an N×N block corresponding to the motionvector MV from the previous frame data output from frame memory 17 tothen supply the read N×N block to a subtractor A1 connected with inputport 10. Then, subtractor A1 obtains the difference between the N×Nblock supplied to input port 10 and the N×N block having the similarpattern thereto supplied from motion compensator 19. The output data ofsubtractor A1 is coded and then transmitted to the receiver side, asdescribed above. That is to say, initially, the video signal of onescreen (intraframe) is coded wholly to then be transmitted. For thevideo signal of the following screen (interframe), only the differentialsignal due to the motion is coded to then be transmitted.

Meanwhile, the data whose motion is compensated in motion compensator 19is summed with the video signal output from inverse DCT means 16 in anadder A2 and is thereafter stored in frame memory 17.

Refresh switches SW1 and SW2 are turned off at a certain interval (here,the period is one group of pictures or a GOP period) by a control means(not shown), so that an input video signal is coded into a PCM mode tothen be transmitted in the case of an intraframe mode and so that onlythe differential signal is coded to then be transmitted in the case ofan interframe mode, thereby refreshing cumulative coding errors for aconstant period (one GOP). Also, a refresh switch SW3 allows thetransmission errors on a channel to deviate from the receiver sidewithin the constant time period (one GOP).

In this manner, the coded image data V_(c) is transmitted to thereceiver side to then be input to the decoding system shown in FIG. 2.The coded image data Vc is decoded through the reverse process to thecoding process in a variable-length decoder 21. The data output fromvariable-length decoder 21 is inverse-quantized in an inverse quantizer22. At this time, inverse quantizer 22 adjusts the magnitude of theoutput DCT coefficients depending on the quantization step size Q_(ss)supplied from the encoding system.

An inverse DCT means 23 inverse-DCT-operates the DCT coefficients of afrequency domain, supplied from inverse quantizer 22, into the imagedata of a spatial domain.

Also, the motion vector MV transmitted from coding system shown in FIG.1 is supplied to a motion compensator 24 of decoding system. Motioncompensator 24 reads out the N×N block corresponding to the motionvector MV from the previous frame data stored in a frame memory 25,compensates the motion and then supplies the compensated N×N block to anadder A3. Then, adder A3 adds the inverse-DCT-operated DPCM data to theN×N block data supplied from motion compensator 24 to then output to adisplay.

FIGS. 3A, 3B and 3C schematically show the process of coding image data.The sampling data of an N×N block shown in FIG. 3A is DCT-operated to beDCT coefficients of a frequency domain by the DCT method, etc., as shownin FIG. 3B. The DCT coefficients are quantized and are scanned in azigzag pattern, to then be coded in the form of runlength andlevel-length, as shown in FIG. 3C.

While the scanning is performed from a low frequency component to a highfrequency component in scanning the N×N block, as shown in FIG. 3C, a“run” and “level” and set as a pair expressed as [run, level], and isthen coded.

Here, the run represents the number of 0's present between coefficientsnot being “0” among the quantized coefficients of an N×N block, and thelevel corresponds to the absolute value of the coefficient not being“0”.

For example, in the case of an 8×8 block, the run is distributed from“0” to “63” and the level varies depending to the data value output froma quantizer. That is to say, if the quantized output value is indicatedas an integer ranging from “−255” to “+255,” the level has a valueranging from “1” to “+255.” At this time, the positive or negative signis expressed by an extra sign bit. In this manner, when a [run, level]pair is set as a symbol, if the run or level is large, the probabilityof the symbol is statistically very low.

Therefore, as shown in FIG. 4, the block is divided into a regularregion and an escape region according to the probability of the symbol.For the regular region where the probability of the symbol is relativelyhigh, a Huffman code is used in coding. For the escape region where theprobability of the symbol is low, data of a predetermined fixed lengthis used in coding. Here, according to the Huffman code, the higher theprobability of the symbol, the shorter the code is set, and vice versa.

Also, the escape sequence ESQ in which data of escape region is coded iscomposed of an escape code ESC, run, level and sign data S, each havinga predetermined number of bits, as expressed in the following equation(1).ESQ=ESC +RUN +L +S  (1)

For example, as described above, if the quantized value is from “−255”to “+255” in an 8×8 block, the escape sequence has a constant datalength of 21 bits in total since the escape code data ESC is six bits,run data RUN is six bits, level data L is eight bits, and sign data S isone bit.

In this manner, according to the conventional variable-length codingmethod, since various extra information is also transmitted togetherwith coded data and the escape sequence set by one variable-lengthcoding table depending on the statistical characteristics of data has aconstant fixed length, there is a limit in compressing data quantity bycoding transmitted data.

Disclosure of the Invention

Therefore, it is an object of the present invention to provide anadaptive variable-length coding method which improves compressionefficiency of data by selecting an optimal variable-length coding tableamong a plurality of variable-length coding tables according to thecurrent scanning position and quantization step size while scanning in azigzag pattern by block type, i.e., inter/intra mode.

It is another object of the present invention to provide a method fordecoding data coded by the above adaptive variable-length coding method.

To accomplish the above object, there is provided an adaptivevariable-length coding method according to the present invention wherebyquantized orthogonal transform coefficients are scanned in a zigzagpattern, are DCT-operated to be [run, level] data and then arevariable-length-coded in a coding system for image data, the methodcomprising the steps of:

-   -   setting a plurality of variable-length coding tables having        different patterns of a regular region and an escape region        according to statistical characteristics of the [run, level]        data;    -   selecting one of the plurality of variable-length coding tables        according to intra/inter mode information of the currently        processed block, zigzag scanning position and quantization step        size; and    -   variable-length-coding the orthogonal transform coefficients        according to the selected variable-length coding table.

In a decoding system for image data, the adaptive variable-lengthdecoding method according to the present invention for decoding datacoded by the adaptive variable-length coding method, comprises the stepsof:

-   -   setting a plurality of variable-length decoding tables having        different patterns of a regular region and an escape region        according to statistical characteristics of the [run, level]        data;    -   inputting intra/inter mode information transmitted from the        coding system;    -   inputting quantization step size transmitted from the coding        system;    -   detecting position information while zigzag-scanning by        accumulating run values of [run, level] data;    -   selecting one of the plurality of variable-length decoding        tables according to the intra/inter mode information,        quantization step size and position information; and    -   variable-length-decoding the data received according to the        selected variable-length decoding table.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a general coding system for image data;

FIG. 2 is a block diagram of a general decoding system for image data;

FIGS. 3A-3C are schematic diagrams for explaining steps of the dataprocessing process according to the apparatus shown in FIG. 1;

FIG. 4 shows a conventional variable-length coding and decoding table;

FIG. 5 is a schematic block diagram of a variable-length encoder forimplementing an adaptive variable-length coding method according to thepresent invention;

FIGS. 6A and 6B illustrate a method for selecting a variable-lengthcoding table partitioned by a predetermined number in the adaptivevariable-length coding method according to the present invention,wherein FIG. 6A represents the intra mode and FIG. 6B represents theinter mode; and

FIGS. 7A, 7B and 7C are histograms [run, level] for each symbol at thefirst, second and Pth regions shown in FIGS. 6A and 6B.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, a preferred embodiment of the present invention will bedescribed with reference to the accompanying drawings.

In the adaptive variable-length coding method according to the presentinvention, a plurality of variable-length coding tables are used. Thetable is selected in accordance with a block type, quantization stepsize and a current scanning position while scanning a block in a zigzagpattern. This selection is in accordance with the statisticalcharacteristics of [run, level] data which vary depending on block type,i.e., intra mode/inter mode or luminance signal/color signal,quantization step size and a current zigzag scanning position, and whichwill be described in more detail.

The inter mode for coding the differential signal between the currentblock data and motion compensated block data generates most of the DCTcoefficients as “0” but scarcely generates larger values, compared tothe intra mode for coding input block image data sequentially. This isbecause the variation in a motion compensation estate error thereof istypically smaller than that of the original video signal.

Also, the statistical characteristics of color which depend on thedecimation in the spatial domain and narrow bandwidth are different fromthose of luminance.

Therefore, in accordance with intra/inter mode and luminance/colorinformation, there may be four block types, i.e., (intra, luminance),(intra, color), (inter, luminance) and (inter, color). However, for theblock type is the present invention, the luminance/color information isexcluded and only the intra/inter mode is considered, because the colorstatistics are dependent on the downsampling structure of the colorsignal.

Also, in the case of a large quantization step size. DCT coefficientsare not high in the high frequency components and many are generated as“0's” while the quantizer scans in a zigzag pattern. That is to say, inorder to utilize the human visual characteristics, the DCT coefficientsare divided into primary weighting matrices. Since the weighting matrixfor high frequency component is large, when the current scanning is ahigh frequency component, small values (including “0”) are oftenproduced but large values are scarcely generated.

Therefore, the present invention proposes an adaptive variable-lengthcoding/decoding method using a plurality of variable-lengthcoding/decoding tables in which the block type (intra/inter mode),scanning position and quantization step size are combined, which iscalled a Huffman code book.

Also, the present invention is adopted for a general coding system shownin FIG. 1 and for a general decoding system shown in FIG. 2.

FIG. 5 is a schematic block diagram of a variable-length encoder forimplementing the adaptive variable-length coding method according to thepresent invention.

According to FIG. 5, quantized DCT coefficients are scanned in a zigzagpattern by zigzag scanner 31.

Variable-length coding table selector 32 outputs a control signal forselecting the corresponding first to Pth variable-length coding tables33.1, 33.2, . . ., 33.P according to the block type (intra/inter mode),quantization step size Qss, and scanning position SP.

The quantized DCT coefficients output from zigzag scanner 31 arevariable-length-coded in accordance with the selected variable-lengthcoding table, to then be transmitted to buffer 14 shown in FIG. 1.

Variable-length decoder 21 of the decoding system shown in FIG. 2variable-length-decodes data coded in the reverse order to that of thevariable-length coding process as shown in FIG. 5.

Subsequently, the method for selecting a plurality of variable-lengthcoding/decoding tables will be described in detail with reference toFIGS. 6A, 6B and 7A to 7C.

FIG. 6A shows P variable-length coding tables T₁, T₂, . . . , T_(p)selected in accordance with quantization step size Q_(ss) and thecurrent scanning position SP (during zigzag scanning) for the intramode. FIG. 6B shows P variable-length coding tables T₁, T₂, . . ., T_(p)selected in accordance with quantization step size Q_(ss) and thecurrent scanning position SP (during zigzag scanning) for the intermode.

The “0” scanning position SP corresponds to the DC component, the “63”scanning position SP represents the last scanning position in thecorresponding block, and quantization step size Q_(ss) has valuesranging from “0” to “62.”

First, in order to select one of P variable-length coding tables T₁, T₂,. . ., T_(p), it is determined whether the currently process block modeis an inter mode or intra mode.

That is to say, as shown in FIGS. 6A and 6B, the blocks for selectingthe variable-length coding tables T₁, T₂, . . ., T_(p) are differentdepending on the mode. In other words, compared to the inter mode, theintra mode has larger selection blocks for the first and secondvariable-length coding tables T₁ and T₂ and a smaller selection blockfor the Pth variable-length coding table T_(p).

In the determined mode, the first, second or Pth variable-length codingtable T₁, T₂ or T_(p) are selected in accordance with quantization stepsize Q_(ss) and scanning position SP.

Quantized DCT coefficients are variable-length-coded in accordance withthe selected variable-length coding table.

Here, an example of P regions partitioned on a (SP, Q_(ss)) plane inaccordance with intra and inter modes shown in FIGS. 6A and 6B can beexpressed as follows.

-   -   In the intra mode:    -   region 1: SP+Q_(ss)<K₁;    -   region 2: K₁≦SP+Q_(ss)<K₂; and    -   region P: K_(p)−1≦SP+Q_(ss)<K_(p)In the inter mode:    -   region 1: SP+Q_(ss)<L₁;    -   region 2: L₁≦SP+Q_(ss)<L₂; and    -   region P: L_(p)−1≦SP+Q_(ss)<L_(P)

The proper partition as above can be sought empirically based onsufficient statistical analysis for various experimental states. Thesestates include such factors as video sequence, bit rate, GOP andpartitioning method.

FIGS. 7A, 7B and 7C show examples of the variable-length coding tablesshown in FIGS. 6A and 6B.

The variable-length coding tables have a regular region and escaperegion which differ depending on the statistical characteristics of[run, level].

That is to say, the first, second, . . . , Pth tables T₁, T₂, . . . ,T_(p) have the regular region and escape region having differentpatterns and the Pth table T_(p) has a smaller regular region than thatof the first or second tables T₁ or T₂.

Meanwhile, the [run, level] symbol is likely to have a low probabilitythereof if the run and/or level lengths have a large value. As shown inFIG. 4, the respective symbols of the escape region has a fixed lengthof 21 bits obtained by adding a six-bit escape code, an eight-bit run,one-bit sign data.

However, in escape coding, since there is redundancy in the run andlevel fields, the data quantity may be reduced. That is to say, the bitnumber required for expressing run is dependent on the scanning positionduring zigzag scanning for two dimensional DCT coefficients and the bitnumber required for expressing level is dependent on the quantizationstep size. Also, quantization weighting matrices of intra-coded blocksand inter-coded blocks are different from each other.

The new escape sequence ESQ having a fixed length of 21 bits can bemodified into that having a variable length using the aforementionedcharacteristics according to Equation (1) above, where ESQ is composedof six bits, RUN is composed of zero to six bits, L is composed of oneto eight bits, S is composed of one bit, the run data is dependent uponscanning position, and the level is dependent upon quantizer.

Therefore, since the modified escape sequence has a variable lengthranging from eight to 21 bits, compared to the fixed length of 21 bits,image data can be further compressed.

In decoding the new escape sequence, since the respective currentscanning positions are automatically matched for the coding system anddecoding system, the number of bits required for expressing the runvalue can be matched without transmitting extra information. Also, inthe case of the level length, since the quantization step size istransmitted to the decoding system for inverse quantization, thetransmitted quantization step size can be used in synchronizing thenumber of bits required for expressing level, which requires no extrainformation to be transmitted.

The above-described variable-length coding and decoding methods whichimprove compression efficiency by adjusting the length of the escapesequence variably are disclosed in the U.S. pat. application Ser. No.08/069,914 filed on Jun. 1, 1993 by the assignee of the presentinvention.

According to the present invention, a plurality of variable-lengthtables are provided for both the coding and decoding sides, which may beslightly more complex in hardware, compared to the case of using aconventional single table. However, the present invention is adopted forthe case when a high data compression rate is necessary. Also, thecorresponding mode, quantization step size and scanning positioninformation generated in coding side is transmitted to the decodingside. The mode and quantization step size information is transmitted ina constant period of time or is transmitted whenever there is a change.The scanning position information is not transmitted separately but isobtained automatically by accumulating the run values after obtaining[run, level] values of the decoding side.

Therefore, although the information on the selected variable-lengthcoding table is not transmitted separately with respect to the blockdata transmitted to the decoding side, the variable-length coding tableselected during coding can be identified from the mode and quantizationstep size information transmitted from the coding side and the positioninformation automatically calculated from the run value in the decodingside. Then, the same variable-length coding table as that adopted forcoding is used for decoding the transmitted block data.

As described above, the method according to the present invention canincrease data compression efficiency such that image data coded anddecoded by selecting one of a plurality of variable-length coding tableshaving a regular region and an escape region, using mode, quantizationstep size and zigzag scanning position information.

Also, according to the present invention, no extra bit which expressesthe variable-length coding table selected during coding is necessary tobe transmitted for decoding. The transmission data can be furthercompressed by adjusting variably the run and level lengths of the datato be coded in the escape region of the selected variable-length codingtable.

Industrial Applicability

An adaptive variable-length coding/decoding method according to thepresent invention can improve the compression efficiency of digitallytransmitted data and is applicable to various technological fieldsincluding digital communication, multimedia and personal computersystems, and digital video apparatuses such as a high definitiontelevision or digital videocassette recorder.

1. An adaptive variable-length coding method whereby quantizedorthogonal transform coefficients are scanned in a zigzag pattern, aremodified into run, level data and then are variable-length coded in acoding system for image data, said method comprising the steps of:setting a plurality of variable-length coding tables having differentpatterns of a regular region and an escape region according tostatistical characteristics of said run, level data; selecting one ofsaid plurality of variable-length coding tables according to intra/intermode information of the currently processed block, zigzag scanningposition and quantization step size; and variable-length coding theorthogonal transform coefficients according to said selectedvariable-length coding table, wherein said selecting step has theselecting range of a plurality of variable-length coding tables havingdifferent patterns of a regular region and an escape region according tosaid intra/inter mode information of the currently Processed block. 2.The adaptive variable-length coding method as claimed in claim 1,wherein said variable-length coding table is selected in accordance withsaid zigzag scanning position and quantization step size within therange determined by the corresponding mode.
 3. The adaptivevariable-length coding method as claimed in claim 1, wherein data ofsaid escape region of said variable-length coding table selected in saidvariable-length-coding step is coded into data having variablerun-length and level-length.
 4. An adaptive variable-length decodingmethod for decoding the data coded by said adaptive variable-lengthcoding method as claimed in claim 1, in a decoding system for imagedata, said decoding method comprises the steps of: setting a pluralityof variable-length decoding tables having different patterns of aregular region and an escape region according to statisticalcharacteristics of the run, level data; inputting intra/inter modeinformation transmitted from said coding system; inputting quantizationstep size transmitted from said coding system; detecting positioninformation while zigzag-scanning by accumulating run values of run,level data; selecting one of said plurality of variable-length codingtables according to said intra/inter mode information, quantization stepsize and position information; and variable-length decoding the datareceived according to said selected variable-length coding table.
 5. Theadaptive variable-length decoding method as claimed in claim 4, whereinsaid variable-length decoding table selecting step has the selectionrange of a plurality of variable-length coding tables having differentpatterns of a regular region and an escape region according to saidintra/inter mode information of the currently processed block in saidmode information inputting step.
 6. The adaptive variable-lengthdecoding method as claimed in claim 5, wherein said variable-lengthdecoding table is selected in accordance with said zigzag scanningposition and quantization step size within the range determined by thecorresponding mode.
 7. The adaptive variable-length decoding method asclaimed in claim 4, wherein data of said escape region of saidvariable-length decoding table selected in said variable-length-decodingstep is decoded into run, level data corresponding to variablerun-length and level-length.
 8. An adaptive variable-length decodingmethod for decoding image data encoded by an adaptive variable-lengthcoding method, in which quantized orthogonal transform coefficients ofthe image data are scanned in a predetermined pattern and encoded, thedecoding method comprising: selecting one of a plurality ofvariable-length decoding tables according to intra/inter modeinformation, scanning position and quantization step size, wherein theplurality of variable-length decoding tables comprise: a tableselectable for an alternating-current (AC) component of an intra modethat is different from a table selectable for an inter mode, and a tableselectable for a direct-current (DC) component of said intra mode; andvariable-length decoding the encoded quantized orthogonal transformcoefficients according to the selected variable-length decoding table.9. The adaptive variable-length decoding method of claim 8 , wherein thevariable-length decoding tables have different patterns of a regularregion and an escape region.
 10. The adaptive variable-length decodingmethod of claim 9 , wherein data of the escape region of the selectedvariable-length decoding table is decoded into data having variable orfixed run-length and level-length.
 11. An adaptive variable-lengthdecoding method for decoding image data encoded by an adaptivevariable-length coding method, in which quantized orthogonal transformcoefficients of the image data are scanned in a predetermined patternand encoded, the decoding method comprising: selecting one of aplurality of variable-length decoding tables according to intra/intermode information, scanning position and quantization step size, whereinthe operation of the selecting one of the plurality of variable-lengthcoding tables has a selecting range of the plurality of variable-lengthdecoding tables, and the plurality of variable-length decoding tablescomprise: a table selectable for an alternating-current (AC) componentof an intra mode that is different from a table selectable for an intermode; and a table selectable for a direct-current (DC) component of saidintra mode; and variable-length decoding the encoded quantizedorthogonal transform coefficients according to the selectedvariable-length decoding table.
 12. The adaptive variable-lengthdecoding method of claim 11 , wherein the variable-length decodingtables have different patterns of a regular region and an escape region.13. The adaptive variable-length decoding method of claim 12 , whereindata of said escape region of the selected variable-length decodingtable is decoded into data having variable or fixed run-length andlevel-length.